<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
<span th:if="${msg}" th:text="${msg}"></span>
<form action="/file_upload" method="post" enctype="multipart/form-data">
  <input type="hidden" name="id" th:value="${id}">
  <input type="file" name="file">
  <input type="submit">
</form>

<form action="/files_upload" method="post" enctype="multipart/form-data">
  <input type="hidden" name="id" th:value="${id}">
  <input type="file" name="files">
  <input type="file" name="files">
  <input type="file" name="files">
  <input type="submit">
</form>
</body>
</html>